Vehicles equipped with a hydraulic booster and an anti-lock braking system or vehicles equipped with an active suspension system have a fluid pressure source apparatus for supplying an operating pressure (high pressure). The fluid pump of the fluid pressure source apparatus is driven by an electric motor or a vehicle engine via a clutch.
A conventional fluid pressure source apparatus is disclosed in U.S. Pat. No. 5,221,125. In this apparatus, fluid from a reservoir is pressurized by a fluid pump driven by a motor and is then discharged into a high pressure line. An accumulator is connected to the high pressure line to accumulate the fluid pressure discharged by the pump.
A high pressure-side pressure switch and a low pressure-side pressure switch are connected to the high pressure line. Pressure detection signals generated by the pressure switches are inputted to an electronic control unit. Both the pressure switches have hysteresis characteristics. When the pressure detection signal indication from the high pressure-side pressure switch turns from high pressure to low pressure, the electronic control unit drives the fluid pump through operation of an electric motor. When the pressure detection signal indication from the high pressure-side pressure switch turns from low pressure to high pressure, the electronic control unit stops driving the fluid pump. Because the pressure (normal lower limit pressure) at which the pressure detection signal indication from high pressure to low pressure is lower than the pressure (normal upper limit pressure) at which the signal indication turns from low pressure to high pressure, the pressure in the accumulator is maintained between the normal lower limit pressure and the normal upper limit pressure.
The low pressure-side pressure switch is used to detect an excessive or abnormal drop of the pressure in the accumulator. If the pressure in the accumulator falls to a level (abnormal lower limit pressure) that is lower than the normal lower limit pressure, the low pressure-side pressure generates a pressure detection signal indicating "low pressure" and the electronic control unit outputs a warning.
As long as the high pressure-side pressure switch is normally functioning so that based on the pressure detection signal therefrom the electronic control unit normally controls the driving of the fluid pump, the accumulator pressure does not fall to the abnormal lower limit pressure. However, if the high pressure-side pressure switch fails, that is becomes unable to output a pressure detection signal indicating low pressure (lower than the normal lower limit pressure), the fluid pump is not driven even though the accumulator pressure is below the normal lower limit pressure. Further, because the electronic control unit does not operate the fluid pump even though the pressure detection signal from the low pressure-side pressure switch indicates "low pressure", the pressure in the accumulator falls below the abnormal lower limit pressure. As a result, a minimum pressure required in the accumulator cannot be achieved.